Explanation: What's happened to our Sun? Nothing very unusual -- it just threw a filament. At the end of last month, a long standing solar filament suddenly erupted into space producing an energetic Coronal Mass Ejection (CME). The filament had been held up for days by the Sun's ever changing magnetic field and the timing of the eruption was unexpected. Watched closely by the Sun-orbiting Solar Dynamics Observatory, the resulting explosionshot electrons and ions into the Solar System, some of which arrived at Earth three days later and impacted Earth's magnetosphere, causing visible aurorae. Loops of plasma surrounding an active region can be seen above the erupting filament in the ultraviolet image. If you missed this auroral display please do not despair -- over the next two years our Sun will be experiencing a solar maximum of activity which promises to produce more CMEs that induce more Earthly auroras.

This is indeed a fantastic APOD!
To be honest, solar images don't really do that much for me. However, after watching this link - especially around the 40 second timeframe (I can't get over how HUGE that CME really is! ) - I can say I have a new appreciation for close-ups of the sun!

<<Proxima Centauri is a red dwarf star about 4.24 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun. It has an apparent visual magnitude of 11, so a telescope with an aperture of at least 8 cm is needed to observe this star even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon. More than 85% of its radiated power is at infrared wavelengths. In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star. Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known. The proximity of the star allows for detailed observation of its flare activity.

Because of the proximity of this star, its angular diameter can be measured directly, yielding a diameter one-seventh that of the Sun. Proxima Centauri's mass is about an eighth of the Sun's, and its average density is about 40 times that of the Sun. Because of its low mass, the interior of the star is completely convective, causing energy to be transferred to the exterior by the physical movement of plasma rather than through radiative processes. This convection means that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core, but is instead circulated throughout the star. Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogen comes to an end.

Convection is associated with the generation and persistence of a magnetic field. The magnetic energy from this field is released at the surface through stellar flares that briefly increase the overall luminosity of the star. These flares can grow as large as the star and reach temperatures measured as high as 27 million K — hot enough to radiate X-rays. The peak X-ray luminosity of the largest flares can reach 1021 W. Since Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could support life is disputed. Nevertheless, because of the star's proximity to Earth, it has been proposed as a destination for interstellar travel.

The chromosphere of this star is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm. About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle. Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona. However, the overall activity level of this star is considered low compared to other M-class dwarfs, which is consistent with the star's estimated age of 4.85 × 109 years, since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases. The activity level also appears to vary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years. Proxima Centauri has a relatively weak stellar wind, resulting in no more than 20% of the Sun's mass loss rate from the solar wind. Because the star is much smaller than the Sun, however, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface.>>

This movie taken by NASA'S Galaxy Evolution Explorer show one of the largest flares, or star eruptions, ever recorded at ultraviolet wavelengths. The star, called GJ 3685A, just happened to be in the Galaxy Evolution Explorer's field of view while the telescope was busy observing galaxies. As the movie demonstrates, the seemingly serene star suddenly exploded once,then even more intensely a second time, pouring out in total about one million times more energy than a typical flare from our Sun. The second blast of light constituted an increase in brightness by a factor of at least 10,000.

You can find a link to the video at the Galex/Caltech address in the quote above.

1) a cavity of low electron density,
2) a dense core (the prominence, which appears as a bright region on coronagraph images embedded in this cavity),
and 3) a bright leading edge.

Most ejections originate from active regions on the surface, such as groupings of sunspots associated with frequent flares. These regions have closed magnetic field lines, in which the magnetic field strength is large enough to contain the plasma. These field lines must be broken or weakened for the ejection to escape from the sun. However, CMEs may also be initiated in quiet surface regions, although in many cases the quiet region was recently active. During solar minimum, CMEs form primarily in the coronal streamer belt near the solar magnetic equator. During solar maximum, they originate from active regions whose latitudinal distribution is more homogeneous.

Coronal mass ejections reach velocities between 20km/s to 3200km/s with an average speed of 489km/s, based on SOHO/LASCO measurements between 1996 and 2003. The average mass is 1.6×1012kg. The values are only lower limits, because coronagraph measurements provide only two-dimensional data analysis. The frequency of ejections depends on the phase of the solar cycle: from about one every fifth day near the solar minimum to 3.5 per day near the solar maximum. These values are also lower limits because ejections propagating away from Earth (backside CMEs) can usually not be detected by coronagraphs.

Current knowledge of coronal mass ejection kinematics indicates that the ejection starts with an initial pre-acceleration phase characterized by a slow rising motion, followed by a period of rapid acceleration away from the Sun until a near-constant velocity is reached. Some balloon CMEs, usually the slowest ones, lack this three-stage evolution, instead accelerating slowly and continuously throughout their flight. Even for CMEs with a well-defined acceleration stage, the pre-acceleration stage is often absent, or perhaps unobservable.>>

Rick M wrote:Huge wow factor. APODs often evoke "ooh" and "ah" from me, but I can't remember the last time I saw the day's picture and literally went "Wow!"

Would this CME have been visible to the naked (well, properly protected) eye if one had looked at the right time, or would the daytime sky have been so bright as to mask it?

Hi Rick. Yes, wow!

SDO observes the Sun in ultraviolet light. All images from SDO are false color, with the different wavelengths and intensities of ultraviolet radiation portrayed with different colors of visible light.

Of course you never want to look directly at the Sun without proper protection. Unfiltered sunlight will damage your retina very quickly, and your retina doesn't have any pain receptors, so the damage would be done before you start hurting. Magnified unfliltered sunlight, e.g. through binoculars or a telescope, will cause instant and irreversible blindness.

To observe the Sun safely you need to filter out all the infrared and ultraviolet radiation, and 99.99 percent of the visible light. One safe way to do this is with number 14 welder's glass. Without magnification the Sun appears half a degree across, about the same size in the sky as the full Moon. With very sharp eyes you might be able to see the largest sunspots on the surface of the Sun, but that's about it.

The easiest and cheapest way to observe the Sun through binoculars or a small telescope is to use a white light filter. You'll see dark sunspots on the bright surface of the Sun. Interestingly, sunspots are cool in the infrared spectrum, dark in visible light, and bright in the ultraviolet spectrum.

Solar afficionados like to observe the Sun through Hydrogen-alpha filters that transmit only one very narrow band of visible red light. This particular frequency of red light is emitted by Hydrogen atoms low in the solar atmosphere when they are excited by solar radiation. In Hydrogen-alpha light solar filaments and prominences can be seen dancing above the surface of the Sun's photosphere, the part of the Sun we see in full-spectrum visible light.

To answer your specific question, to observe a coronal mass ejection in visible light you need to block all the light from the disk of the Sun with a coronagraph, essentially creating an artificial solar eclipse. CME's are mostly electrons and protons, and as they travel away from the surface of the Sun they quickly become too diffuse to reflect enough light to be seen. Space weather forecasters infer the path of a CME through the solar system from its initial trajectory and from the much faster electromagnetic radiation emitted when the CME first erupts.

Lots of people have been wowed by this picture, for good reasons. To me, the greatest wow about the Sun is how calm, quiet and well-behaved it is. Most other stars similar to the Sun appears to be more variable, i.e. prone to more and greater outbursts, than the Sun.

http://en.wikipedia.org/wiki/Kepler_(spacecraft) wrote about the problems associated with finding planets crossing the disks of other stars:
Most of the additional noise appears due to a larger-than-expected variability in the stars themselves (19.5 ppm as opposed to the assumed 10.0 ppm)

The 10.0 ppm is likely the solar value, and other stars were expected to be like the Sun. Instead they appear to be, on average, twice as noisy.

Another testimony to the remarkable calmness and "polite behavior" of the Sun is how almost-perfectly round it is. See http://asterisk.apod.com/viewtopic.php?f=31&t=29320. Many other stars, by contrast, are very noticeably flattened or - in the case of red supergiants - totally misshapen.

No doubt that mildness is a factor in the rise of life here on Earth to a near-intelligent level. I suppose the relative noisiness of a star is yet another factor that should be added to a more complex and useful Drake equation.

rstevenson wrote:No doubt that mildness is a factor in the rise of life here on Earth to a near-intelligent level. I suppose the relative noisiness of a star is yet another factor that should be added to a more complex and useful Drake equation.

A factor, yes, but not necessarily a positive one. There are good arguments for a dynamic environment being important to the development of a rich biosystem. With a somewhat more active star, complex life might have developed much faster on Earth.